Various forms of apparatus have been developed in the electronic manufacturing arts for automatically placing components on circuit boards. Such apparatus includes a placement machine having a releasably attached placement head assembly, a representative example of which may be seen depicted in accompanying FIGS. 1-3 which are simplified views of the assembly for placement machine model nos. MPS118, MPS500, MPS318, and MPS525 manufactured by the Dynapert Division of the Emhart Machinery Group, Beverly, Mass.
The function of such a head assembly is to automatically grasp a component from a supply thereof, align the component in proper orientation with respect to a site on the board, move it to the desired site and thence place it thereon for subsequent bonding by means by wave or vapor soldering or the like.
With more particular reference to the construction and operation of such a head assembly, in FIG. 1 a placement head 10 is illustrated therein as well as an actuator 12 which, although not an integral part of the head assembly 10, is included as part of the component placement apparatus and co-acts with the assembly to achieve the aforementioned function. The actuator 12 provides motive force in the direction of arrows 14 and 16 which is transferred by actuating lever 18 (pivoted about pivot point 20) to the head assembly 10. More particularly, movement of the actuator 12 in the direction of arrows 14 and 16 produces corresponding opposed motion in the direction of arrows 24 and 22, respectively, by pivoting action of the lever 18 about pivot point 20. The lever 18 includes a bearing 26 which engages an upper surface 28 of a flange 30 portion of an inner tweezer actuating sleeve 32. This sleeve 32 is disposed about a circular main shaft 34 which is, in turn, connected to the placement apparatus.
A similar actuator-lever mechanism is provided (omitted for clarity) having a similar bearing contacting the upper surface 33 of the flange 36 portion of an outer tweezer actuating sleeve 38. The main shaft 34, inner and outer sleeves 32 and 38, respectively, are in coaxial sliding engagement with one another.
Four tweezer arms 40 are pendantly disposed in quadrature about the shaft 34 although in FIG. 1 only three such tweezer arms 40 are visible. The tweezer arms 40 are pivotably connected to the shaft 34 by means of pivot points 42. With reference to the leftmost tweezer arm 40 as an example, such pivoting about point 42 results in the upper and lower portions of the arm 40 moving in opposing directions of the arrows 44 and 46 respectively about the pivot point 42 or conversely in the direction of arrows 48 and 50 respectively.
Each sleeve 32 and 38 is provided with a pair of diametrically opposed bearings. Thus, with further reference to FIG. 1 and the inner sleeve 32, these tweezer actuating rollers 52 may be seen in contact with corresponding upper roller surfaces of respective tweezer arms 40. In like manner to the tweezer arms 40 only three such rollers 52 may be seen.
Upon downward urging of inner sleeve 32 by means of previously described movement of the actuator 12 and lever 18, the rollers 52 will roll on surfaces 54 of the tweezer arms 40 as they move downwards urging the upper portions of the tweezer arms 40 in a radially outward direction of arrow 44. This in turn causes movement radially inwards in a direction of arrows 46 by the tweezer arms 40 to orient a component disposed centrally thereof. Similar downward movement of the outer sleeve 38, also causing rolling engagement of roller 52 with correlative surfaces of the upper portions of tweezer arms 40 also causes such radially inward movement of the remaining pair of tweezer arms. A spring 56 is disposed circumferentially about and in engagement with the upper portions of the four tweezer arms 40 biasing these upper tweezer arm portions radially inwards. In this manner after the aforesaid radially inward movement of the lower portions of tweezer arms 40 causes by the downward movement of the sleeves 32 and 38, the spring 56 will cause a restoring force radially inwards on the upper portions of the tweezer arms 40 to restore the arms 40 to the positions shown in FIG. 1 prior to a next downward urging of the sleeves 32 and 38.
With reference to FIG. 2, the head assembly 10 is typically provided with a plurality of tweezer heads 60, each comprised of a jaw adjusting block 62 and tweezer jaw 64 disposed on a respective lower end of its corresponding tweezer arms. The block 62 is attached to the lower end of each respective arm 40 by means of a screw 66 through a slot 68 in the block 62. The tweezer jaw 64 is retained in the jaw block 62 by means of a screw 68 extending through screw hole 70. An inner gripping surface 72 is thereby presented radially inward for aligning contact with the component.
The general sequence of operation of such a previously known placement head assembly 10 may be seen with reference to FIGS. 3A-3G. With reference to FIG. 3, an additional component of the assembly 10 not hereinbefore discussed may be noted, namely a vacuum support means 74 interconnected in coaxial sliding engagement within the hollow main shaft 34 and movable in the direction of the longitudinal axis of such shaft 34 and vacuum support means 74. A plurality of components 76 to be placed on circuit board sites are conventionally provided carried by a roll of tape. In the operation depicted in FIG. 3, the vacuum support means 74 is lowered so as to contact the upper surface of one of the components 76 whereupon a vacuum is drawn through the vacuum support 74 so that the component may be temporarily suspended from the end of the vacuum support 74 as shown in FIG. 4.
Upon the downward movement of the sleeves 32 and 38 as a result of the motion provided by the actuators 12, the tweezer arms 40 are caused to move radially inwards in a manner previously described whereby the respective inner component contacting surfaces 72 of each tweezer head 60 contacts a respective side of the component 76 in quadrature so as to align the component as desired in a plane normal to the longitudinal axis of the head assembly 10 for subsequent placement on the circuit board site. Such inward movement of the tweezer heads 60 may be seen by a comparison of their position in FIG. 4 with that of their subsequent position in FIG. 5. Inasmuch as the component is only marginally supported at this point by means of the vacuum support 74, the component is free to move about on the end thereof whereby contact with the jaws 64 will cause the component to be oriented as defined by the inward contact of the jaws 64 with the component sides.
Comparison of FIGS. 5 and 6 will indicate that the head assembly 10 may thence be rotated in any manner desired and moved laterally by various mechanical linkages, robotics arms or the like so as to position the component 76 as shown in FIG. 7, with the component vertically adjacent a desired site 78 on a circuit board 80. This site 78 may include deposition of epoxy dots as desired so as to cause the subsequently placed component to adhere thereto. It may further be seen from FIG. 7 that as a result of return movement of the actuators 12 in the direction of arrow 16 and the inward biasing action of the spring 56 on the upper portion of the tweezer arms 40, the tweezer heads 60 are thence moved radially outwards away from the vacuum support 74 and component 76. In FIG. 8, the vacuum support 74 has been urged downwards so as to cause the component 76 to contact the site 78 on the board 80. Finally, in FIG. 9 the vacuum has been terminated in the vacuum support 74 hwich has thence been urged upwards leaving the component 76 deposited on the site 78 as desired.
With the foregoing description completed of general operation of the placement head assembly 10, several serious deficiencies in the design thereof may now be noted which have been eliminated by the instant invention.
First, due to the coaxial sliding engagement between the main shaft 34, inner and outer sleeves 32 and 38, several problems have quite frequently arisen. Because of the leverage effect of the tweezer arms 40 lateral displacement of the sleeves 32 or 38, due to play between the various moving components (particularly the sleeves) may result in imprecise and inconsistent movement of the jaws 64 and their contacting surfaces 72 and their final radially inwardmost positions. In that this final position dictates the precise final alignment of the component 76 for subsequent placement on the circuit board 80, such play results in inaccurate placement of the component on the circuit board site. With increasing chip densities and smaller components, the problem is exacerbated due to need for increasingly more precise component placement. Lateral positioning inaccuracies of the sleeves 32 and 38 may be multiplied by as much as a factor of 12 in the position of the inner jaw contacting surfaces 72 by reason of the aforementioned leverage effect of the tweezer arms 40.
Improving closeness of the fit between the sleeves 32, 38 and shaft 34 may control the problem but have associated higher manufacturing cost associated therewith as well as maintenance problems in maintaining the sliding surfaces in a smooth, polished or oiled state.
Increased friction or variation in friction caused by closer tolerances creates an additional problem in controlling the amount of aligning force imparted to the components 76 by the jaws 64. The jaws 64 typically engage connector leads on the outer periphery of the components 76 during the aligning process. Circuit density increases result in smaller components with attendantly smaller and more fragile circuit leads and housings which in turn call for previse control over the forces applied inwardly to the component. Such control is rendered difficult by the sliding mechanism of the sleeves 32 and 38. Undesired deformation of the components 76 is often caused by yet an additional and related design deficiency of prior head assemblies. In order to maintain smooth sliding surfaces the sleeves 32 and 38 are typically machined of a dense metal such as brass or the like resulting in collars or sleeves with relatively large inertias imparted through the tweezer arms 40 to the components 76. This results in undesirable excessive impacts of the jaw heads on the components leading to destruction of the components, crushing of the leads thereof, and the like. Moreover, the upward restoring force to these heavy sleeves 32 and 38 must be completely provided by the retention spring 56 thereby giving rise to additional problems.
Yet an additional and even more serious problem related to prior head assemblies 10 results from the fact that the actuators 12 are integral parts of the placement machine itself and separate from the head assembly 10. Consequently, mechanical linkage between the actuators 12, actuating levers 18 and head assembly 10 is only completed for final calibration and adjustment when the head assembly 10 is on the machine. Accordingly, expensive down times were associated with replacement of head assemblies inasmuch as such alignment and adjustment of the head assembly had to occur in situ when in place in the machine thereby reducing the productive time of such expensive placement machines, typically costing hundreds of thousands of dollars. Due to the relatively complex linkages associated with the placement machine and head assemblies, no means was readily available for off-line adjustment and alignment of the head assembly so as to minimize down time of the placement machine during head assembly replacement.
For the foregoing and other reasons, a placement head assembly was desired which was simpler, more reliable, less expensive and non-destructive of the component sought to be placed, and which could significantly reduce placement machine down time during head assembly replacement while providing for off-line adjustment and alignment thereof.